when you press forward on the d-pad, your horse begins to walk, tapping A, he/she will begin a trot, pressing A again, the trot will quicken, pressing A again, the horse will begin a gallop. To make your horse sprint, quickly press A repeatedly. Doing this in time with the horse will slow the stamina drain. I've heard that doing this in time with the horse will stop all stamina drain, but I have not seen that and I've tried this over and over. Full on running with your horse will drain stamina, but timing the A button presses with your horses gallop (also take into account the grunting, this helps as well) will make the stamina drain much, much less.
So it should look like this:
begin walking-->Press A--->horse beings slow trot--->press A--->horse begins quicker trot---->press A---->horse begins running--->repeatedly pressing A causes your horse to being sprinting. The faster you press A, the faster the horse goes, up to its maximum speed. Timing the A button presses will slow stamina drain.
As to your other points:
Yes, your horse is pretty lazy when you call it. It does take some time and it never really comes right to your feet. Just like any real horse I've been around, they aren't exactly golden retrievers.
Yes, focusing on something while riding causes the camera to do some very whacky things. I've hit my fair share of trees, most recently while running from a grizzly bear that wanted me for dinner.
Yes, the AI is faster, aims better, and seems to have perfect horse control while we are left panning around like crazy, shooting all over the place, and in general hamfisting the controller.
I can't and won't argue with those points, but I can say that they are issues caused by two things that I personally have done myself, so I am comfortable in suggesting methods to learn:
1. Don't focus on people when going any faster than a trot. If you are going fast, the camera goes crazy. So, if you are going to focus on someone or something, make sure you are trotting.
2. When shooting, it's best to use the auto aim feature. Get your cursor near the enemy and pull the left trigger and it should quickly move to the bad guy. This only lasts for a second or two. You must be quick in firing.
As for your horse not following you when you are driving something for a mission, I personally haven't had issues with this. I also haven't been bucked off during firefights. I have have several run ins with wolves as well and my horse has remained well composed during the brief battles but I've also gotten quite good at dispatching wolves; they get the two barrel hello.
I do admit, controlling your horse is difficult and takes practice. Spend some time in a field practicing.
Finally, I also have not had the issues with cinematic view that I have seen on Facebook/Youtube, or read about here. I set a marker on my map, begin riding and press the menu button long enough to go into cinematic camera and I've not yet run into a wagon/train/tree/or other horse. I have however, done all of these manually. In fact, I killed another rider by slamming into him coming over a crest. Oops.